1. Field of the Invention
Embodiments of the present invention generally relate to the field of light-emitting diode (LED) technology and, more particularly, to a vertical light-emitting diode (VLED) structure with increased light extraction.
2. Description of the Related Art
Luminous efficiency of LEDs can be defined as the total apparent power of a light source to its actual total input power (luminous flux divided by input power). Having units of lumens per watt, luminous efficiency measures the fraction of power which is useful for lighting. As a type of light source, light-emitting diodes (LEDs) have been designed and developed over the past few decades to make improvements in luminous efficiency and increase the number of possible applications for these solid state devices.
Beginning with a conventional LED structure whose cross-section is shown in FIG. 1, one can see why the luminous efficiency of these devices is relatively poor. A conventional LED 100 is formed on a substrate 104 such as sapphire, silicon carbide, silicon, germanium, ZnO or gallium arsenide depending on the composition of the LED layers to be deposited. An n-doped layer 102 is disposed above the substrate 104, and this layer 102 may comprise n-doped GaN. GaN may be grown on a sapphire substrate for emitting green to ultraviolet (UV) wavelengths of light. A multiple quantum well (MQW) active layer 103 is deposited above the n-doped layer 112, and this is where photon generation occurs when the diode is properly biased. A p-doped layer 106 is grown above the active layer 103 in FIG. 1. After portions of the p-doped layer 106 and the active layer 103 are removed to expose a portion of the n-doped layer 102, electrodes 108 and 110 may be formed on the p-doped and n-doped layers, respectively, for forward biasing the LED.
To improve upon some of the design limitations for luminous efficiency of conventional LEDs, the vertical light-emitting diode (VLED) structure was created. The VLED earned its name because the current flows vertically from p-electrode to n-electrode, and a typical VLED 200 is shown in FIG. 2. To create the VLED 200, an n-doped layer 102 is deposited on a substrate (not shown), and this may comprise any suitable semiconductor material for emitting the desired wavelength of light, such as n-GaN or a combination of undoped GaN and n-GaN. A multiple quantum well (MQW) active layer 103 from which the photons are emitted is grown above the n-doped layer 102. A p-doped layer 106 is deposited above the active layer 103 in FIG. 2. A metal layer 202 may be deposited above the p-doped layer 106 for electrical conduction and heat dissipation away from the VLED.
Unwanted dislocations 112 may form in an LED during the growing of one or more of the layers that make up the LED. In conventional LEDs, current flows along the surface very far from the interface of the substrate 104 and the n-doped layer 102 so the effects of dislocations on current are not obvious. Unwanted dislocations may also occur in a VLED, and because the dislocations in a VLED may run in the direction of the current, reductions in the dislocation density may have a more noticeable effect on decreasing the leakage current. Leakage current, as defined herein, generally refers to the current measured when −5V of reversed bias is applied to the LED electrodes.
Accordingly, what is needed is a light-emitting solid state device with reduced dislocation density and increased luminous efficiency.